The present invention is directed to a method and apparatus for treating water by flocculation and/or crystalline-precipitation type treatments. The present invention is particularly directed to such a method and apparatus wherein sludge is recycled and wherein a mixture of water to be treated, sludge and possibly chemical reagents are passed into a decanter. The method and apparatus of the present invention are particularly useful when employing decanting of the laminar type. The method and apparatus of the present invention are additionally particularly suitable for treating water which is charged or saturated with inorganic salts that can be precipitated therefrom in the form of a crystalline sludge by means of known chemical reagents with which they form compounds of low solubility. Such known reagents may for example be lime, soda, baryta, and other known such crystalline-precipitation compounds.
It is known that the inner recycling of sludge strongly accelerates the flocculation and/or crystalline-precipitation process. There are known various installations, generally referred to as solids contact units including a central area wherein mixing and reaction are performed in the presence of sludge. Such units include a decanting area, and the mixture is passed from the central area into the decanting area. Such known devices normally include a central reaction zone and a surrounding annular decanting or clarification zone.
It is further known in such installations that the use of parallel inclined plates, for example inclined at an angle of approximately 60.degree. C. to the horizontal, within the decanting zone achieves an efficient laminar decanting with increased velocities.
However, such previously known installations generally suffer from the following specific operational disadvantages.
Generally, it is difficult to economically arrange a set of parallel inclined plates within an annular area without wasting a portion of the surface area of the decanting zone. This difficulty has to some degree been overcome by placing a circular reaction zone within a rectangular decanting zone, and the sludge which settles in the bottom of the decanting zone is returned to the central reaction zone. However, in such an arrangement sludge would tend to settle in corners and could not be removed therefrom. To overcome this disadvantage it has been necessary to form the decanting zone to include a lower circular area and an upper rectangular area. However, the structural complexities of such a system should be readily apparent.
Additionally, it is known that when the water is being treated by a crystalline-precipitation treatment, and when such treatment is performed in a sufficiently concentrated sludge medium, precipitation takes place on the sludge, the sludge thus attracting the crystalline precipitates. That is, it is known that sludge must be present in sufficient quantities to avoid separate precipitation of the crystals. Such separate precipitation of the crystals would lead to rapid incrustation of the inner elements of the installation. In known systems wherein the reaction zone is located centrally within a decanting area, the concentration of sludge normally decreases from the center towards the periphery of the installation. If the precipitation reaction is not ended when the mixture passes from the reaction zone to the decanting zone, the precipitation reaction is continued within the decanting zone, and if the concentration of sludge at certain areas therein is insufficient, a very rapid and progressive incrustation of the plates within the decanting zone will occur. Such incrustation is magnified due to the fact that the incrustations retard the descending flow of sludge between the inclined parallel plates. Thus, there may rapidly occur inequalities in the spacing between adjacent plates, thereby manifestly rendering the operation of the decanter inefficient.
Further, in such known installations, wherein the mixture is distributed in a radial direction from the reaction zone to the decanting zone, it is substantially impossible to achieve an actual uniform distribution of the mixture throughout the entire area of the decanting zone.
In installations of this type, distribution of the mixture from the reaction zone to the decanting zone is achieved by a pressure drop across the distribution system, i.e. normally across a plurality of openings which feed into the decanting zone. In known installations however, it is possible to achieve a reasonably uniform distribution throughout the entire area of the decanting zone only when operating the installation at approximately the designed nominal flow rate of the installation. That is, if the installation is operated at a substantially reduced rate, it is difficult to achieve or maintain satisfactory fluid pressures within the system to provide a uniform distribution throughout the area of the decanting zone. More paraticularly, an installation must be designed such that when it operates at the designed nominal flow rate the velocity of mixture entering the decanting zone must be maintained at a level sufficiently low to avoid any turbulence within the decanting zone. For example, it is generally considered that the velocity of mixture actually entering a decanting zone may not exceed approximately 1.4 meters per second. This velocity would correspond essentially to a pressure drop across a distribution system of 10 centimeters of water. This would generally be sufficient to provide a uniform distribution throughout the area of the decanting zone. However, if it becomes necessary to operate such an installation at half the designed nominal flow rate, then the pressure drop across the distribution system into the decanting zone would be only approximately 2.5 centimeters of water. Such a pressure drop is clearly insufficient to achieve a uniform distribution of the mixture throughout the area of the decanting zone.